**3.3 Laminar vs. turbulent flow**

138 Biogas

The calorific value is generally used for the billing, as the final consumer/customer must receive his bill with the energy value included, meaning in the unit of kWh in a period (i.e. a year, a month). The energy value yields from multiplication of accumulated flow and

Gas mixing occurs when streams of different gas qualities are united to a single stream. In pipeline systems this means that different gas sources meet in a T-type or Y-type of pipeline. A simple example of two streams of volume (V) with two calorific values (H) is given below (see figure 3). The resultant value depends on the product of volumes or flow (Q) and the

> H��� <sup>=</sup> V� ∗ H� + V� ∗ H� V� + V�

> > = ∑ �Q� ∗ H�� � ∑� Q�

H� <sup>=</sup>∑ �V� ∗ H�� � ∑� V�

Fig. 3. Mixing and dynamic tracking of gas parameters at point K3

12.4

12.8 ... 15.7 15.0

Value Shorthand Unit Group **L** Group **H** 

WS,n kWh/m3 10.5 ... 13.0

Calorific Value HS,n kWh/m3 8.4 ... 11.0 10.7 … 13.1 Relative Density dn - 0.55 … 0.75 0.55 … 0.75

calorific value, e.g. 3000 m3/a \* 10.1 kWh/m3 equal to 30300 kWh/a.

Wobbe-Index Nominal Value

Table 1. Essential gas parameters

**3.2 Basic equations for mixing** 

H1,2 , Hs = resulting calorific value V1 , V2 = volume of stream #1, #2 H1 , H2 = calorific value of stream #1, #2

Qi = flow of stream i ( = dV/dt)

amount of each calorific value according to:

Streams of gas and fluids show a typical behaviour when the flow changes from laminar state to the turbulent state (see figure 4). The areas of the flow type are described by the Reynold number (Re). Beyond Re= 2300 the flow switches suddenly from laminar to turbulent. But he transition spreads over a range and is dependent on physical and technical parameters. In pipeline systems the main parameters are pipeline inner diameter and roughness value which determine the flow type inside. Generally spoken, smaller diameters and higher roughness values lead to turbulent flow (but also to a higher pressure loss which is not really wanted).

(*Re* is defined as*: Re = v d / ʋ* , (*v* = velocity, *d* = pipe diameter, *ʋ* = dynamic viscosity))

Fig. 4. Laminar and turbulent flow in Reynold number vs. pipe roughness number (source /1/)

#### **3.4 Gas conditioning, process optimization**

Gas conditioning is required in biogas plants in order to provide gas of near or equal quality of natural gas when it is fed into the network. Many consumers, especially industrial

Gas Quality Parameter Computation in Intermeshed Networks 141

The data types and sources are summarized, collected and described in the following table

Geogr. Information System (GIS)

GIS SCADA

PGC

The network model must be as actual as possible and should be updated whenever changes occur in reality. Very sensible with regard to the computation result is the information of actual or historical valve positions (open/closed); its tracing is indispensible, because wrong position will cause wrong/deviating results. The size of the network model may extend from some 1000 pipes up to 700 000 pipes (transport system to large distribution system including the transport system). The pressure range of larger networks may go down in

The data sources of different systems and their type of data which are necessary to build up a network model for simulation is shown schematically below (see figure 5). When computing the calorific value for each node (geographic position x,y) over time (t) the

GIS 1 m

SCADA 0.001 bar

0.1 - 1 m3

SCADA, EDM 0.1 m3 per hour,

° C (1 m3)

0.1 mm 1 m 0.01 mm actual as possible

hour, minute

per hour per hour

per month, per year

per hour

Data Type Data Detail Source Accuracy Time Scale

**4.1 Data types, accuracy, positioning, time scale/acquisition cycles** 

to give a short overview:

Control equipment

Physical State

Other/ Derived

Pipe Inner diameter, Length, Roughness, Material class, …

Medium Gas density,

Consumer Flow (output),

Node Geographic (schematic) coordinates,

node type (branch, …)

Valve (open/closed) Regulator (pressure/flow control operation mode), (max./min. flow, pressure)

Gas temperature, Law of pressure loss

Pressure Flow (intake)

Type: RLM, SLP

Consumption history/forecast, planned intake flow

Outside air temperature,

Table 2. Data types required for control and simulation

resulting value will be used to support the billing process.

several levels from 100 (84) bar to 0.020 bar finally at distribution level.

consumers need gas with stable calorific value and/or Wobbe-Index. The allowed tolerance in Germany/Europe is ± 2 %, only.

By means of propane gas which has a higher calorific value (28.1 kWh/m3, approx.) the biogas will be mixed (conditioned) by special equipment to an appropriate final calorific value. The final value is selected and continuously controlled by the network operator depending on gas type (H or L), network structure, flow situation and consumer requirements.

The conditioning of gas is a high extra cost for the network operator (operational cost and propane, especially). An optimization of gas conditioning means that propane gas usage and cost should be minimized. This can be achieved in the conditioning equipment by selecting and setting the set point of the final calorific value to an acceptable low value. But this value must still be compatible with the calorific values in the surrounding network which - of course - must be known. Appropriate measurements, the mixing equations and simulation help to solve the optimization and set point problem, even on-line.

In gas network the gas flow may have a number feeder points – including different gas qualities - and in addition the intermeshed network structure contains potential mixing points at every pipe crossing or branch. Normally, insight of the gas flow and calorific value at certain interesting points in the intermeshed network is only possible by many measurements (e.g. gas chromatography) and/or network computation and simulation.
